In general, the present invention relates to a solid-state image sensing apparatus and its driving method. In particular, the present invention relates to a solid-state image sensing apparatus suitable for use in equipment such as an electronic still camera, and a driving method of the solid-state image sensing apparatus.
In a conventional solid-state image sensing apparatus like, for example, a CCD (Charge Couple Device) solid-state image sensing apparatus employed in an apparatus such as a video camera, electric charge is accumulated in each sensor unit 101 for 1/60 seconds, a period of time corresponding to one field, and pieces of signal electric charge read-out from two adjacent sensor units 101 representing two adjacent picture elements arranged in the vertical direction are combined in a vertical CCD 102 in order to generate an output signal of an interlace system as shown in FIG. 12. In addition, interlace scanning is implemented by adopting a so called field read-out system whereby two adjacent sensor units 101 are combined for an even field in a way different from that for an odd field as shown by right and left diagrams of the figure.
In the field read-out system, the time for accumulating electric charge in a picture element is 1/60 seconds. In a so-called frame read-out system, on the other hand, signal electric charge of only an even line or signal electric charge of only an odd line is read-out during an accumulation time of 1/30 seconds as shown in FIG. 13. Since the accumulation time of the field read-out system is half that of the frame read-out system, the field read-out system offers a merit that the image of a dynamic picture can be well picked up. Since pieces of signal electric charge of two adjacent picture elements in the vertical direction are mixed, however, the field read-out system has a demerit that the vertical resolution is low. Thus, the field read-out system is not an image pickup system appropriate for an electronic still camera which requires a high resolution.
For the reason described above, an electronic still camera employs a solid-state image sensing apparatus of the frame read-out system which uses a mechanical shutter as an image sensing device. As an alternative, an electronic still camera may employ a solid-state image sensing apparatus of a so-called all pixel read-out system whereby pieces of signal electric charge of picture elements are read-out independently of each other without mixing them in vertical CCDs as shown in FIG. 14. The use of the solid-state image sensing apparatus of these systems allows the vertical resolution to be prevented from deteriorating. However, the time it takes to output a pickup image signal is twice that required by the solid-state image taking apparatus of the field read-out system for the same number of picture elements.
In many cases, an electronic still camera is provided with a monitor for displaying a picked-up image of a picture such as a liquid crystal display monitor. Such a monitor facilities adjustment of the focus and the camera angle at an image pickup time. In general, a liquid crystal display monitor displays a picture at a non-interlace scanning speed of 1/60 seconds. As a result, supplying a pickup image signal of a 1/30 second period to the liquid crystal display monitor as it is will give rise to a problem of a distorted display picture like the one shown in FIG. 16.
In order to avoid the problem described above, a V (video) RAM (frame memory unit) 112 is provided for a liquid crystal display (LCD) monitor 11 as shown in FIG. 17. The frame memory unit 112 is used for converting the frame rate. To put it in detail, in the configuration of a display system employing a VRAM unit 112, a pickup image signal with a period of 1/30 seconds is supplied to the VRAM unit 112 while a non-interlace signal with a period of 1/60 seconds is read-out from the VRAM unit 112 to be fed to the LCD monitor 111.
Having a high vertical resolution as described above, a solid-state image sensing apparatus of the all-pixel read-out system is suitable for use in an electronic still camera as an image sensing device. On the other hand, since a VRAM unit or a frame memory unit is required for displaying a taken image of a picture on an ordinary television monitor, there is raised a problem that the cost of an electronic still camera rises. In addition, since an electronic still camera employs automatic control apparatuses such as an automatic focus control apparatus, an automatic iris control apparatus and an automatic white balance control apparatus, there is also raised a problem that the responses of these automatic control apparatuses are made slower by a long period of a signal output by the image pickup device.
As a way to solve the problems described above, a technique of increasing the data rate of a signal output by the image sensing device can be thought of. In order to increase the data rate of a signal output by the image sensing device, however, it is necessary to provide a sampling rate converter. In addition, as the clock frequency is increased, there are also raised new problems such as an increase in power consumption, higher costs of components in use and deterioration of the S/N ratio. Thus, the technique of increasing the data rate of a signal output by the image sensing device can not be said to be a good solution.